The present invention relates to a road transmission equipment used in a communication system between a road and a vehicle, allowing mobile communication between a road and a mobile station by locating a plurality of road antennas along the road to form a cell on the road.
There is an increasing demand for communications between road controllers and vehicles. On a superhighway, in particular, to enable a vehicle to operate on the road without any burden on the driver and any accident both for the controller and the driver, frequent interchange of information is necessary between the road and the vehicle. One type of such a developed system is a self-operating system that allows a vehicle to run with close communication between the road and the vehicle, which are equipped with various sensors and a camera (see, for example, Japanese Unexamined Patent Publication No. 241495 of 1996).
For the construction of a driving support system (hereinafter, referred to as xe2x80x9ccommunication system between a road and a vehiclexe2x80x9d) which makes use of the communication with the vehicle for future extension into a self-operating system, it is necessary to provide a communication area (cell) on the road.
To provide such a cell, we may consider laying a leakage coaxial cable along the road. However, the drawback of this method is that large-scale construction is needed for laying such a cable. In addition, since it is required to locate the leakage coaxial cable at a relatively low position on the ground, the distance for which a radio wave propagates in a direction across a traffic lane is disadvantageously short.
On the other hand, if the communication is performed with a plurality of road antennas being arranged on the road at predetermined intervals, a single road antenna can cover a relatively large cell. In this case, each of the road antennas is connected to a central base station of the road controller via an optical fiber and the like.
In the case where the road antennas are provided, when a large-size vehicle comes proximate to a small-size vehicle, it obstructs the view of the driver of the small-size vehicle, preventing him from seeing the road antenna from inside the small-size vehicle. In particular, it is likely that a microwave or a millimeter wave of a high frequency having a small angle of diffraction is blocked. Accordingly, the communication between the vehicle and the road is interrupted, thereby preventing continued communications.
Therefore, in order to enable continuous communications between the road and the vehicle, multi-station communication has been proposed According to this multi-station communication, a plurality of road antennas having an inherent directivity are provided along the road, and radio waves of the same frequency and the same content are emitted from the respective road antennas toward the same cell.
A multi-station communication system is advantageous because such a system has a plurality of propagation paths for radio waves to be emitted and therefore the radio wave avoids being blocked so as to continuously perform smooth communication between a mobile station and a road communication station even when a vehicle runs proximate to a large-size vehicle such as a truck.
In the multi-station communication system, however, the Doppler effect occurs when a vehicle moves. The antennas receiving radio waves from the front and behind receive radio waves of respectively different frequencies based on Doppler shift.
FIG. 9(a) shows an arrangement of conventional road antennas a, b, and c in a multi-station communication system and a vehicle running under these antennas. A receiving antenna 61 and a receiving device 4 are mounted on the vehicle.
FIG. 9(b) is a graph showing the transitions of deviations of the frequencies received by the receiving antenna 61. The transition of a deviation of the frequency received by the receiving antenna 61 from the road antenna a is indicated by a line a; the transition of a deviation of the frequency received by the receiving antenna 61 from the road antenna b is indicated by a line b, and the transition of a deviation of the frequency received by the receiving antenna 61 from the road antenna c is indicated by a line c.
An exemplar value of the deviation of the frequency received from the road antenna a will be given. Doppler shift xcex94f is expressed by: xcex94f=f0 v/c (c is a velocity of light), where a transmitted frequency of the road antenna is f0, and v is a velocity of a vehicle. When the vehicle runs on the road, the Doppler shift xcex94f is given by:
xcex94f=f0(v/c)L(L2+H2)xe2x88x92xc2xd
where the height of the road antenna from the ground is H, and a distance between the vehicle and the road antenna is L. Assuming f0=5.8 GHz, v=100 km/h, and H=10 (m), the Doppler shift xcex94f is expressed by:
xcex94f=537xc2x7L(L2+H2)xe2x88x92xc2xd(Hz)
When the distance between the road antennas is set to 50 (m), the value of L ranges from 0 (m) to 50 (m). Accordingly, the Doppler shift xcex94f ranges from 0 to 527 (Hz). At the middle point between the antennas, i.e. L=25 (m), the Doppler shift xcex94f is 499 (Hz).
With the arrangement of the road antennas as shown in FIG. 9(a), there occurs skipping of received frequencies as shown in FIG. 9(b) each time a vehicle passes near the middle point between the antennas. This skipping is caused because the automatic frequency control (AFC) of the receiving device 4 is drawn toward the frequency having greater receiving power. This skipping makes it difficult to follow the frequency control in a receiving section, resulting in the interruption of communication during the occurrence of skipping.
Accordingly, it is desired to reduce the Doppler effect on the side of the road-transmitting device in the communication system between a road and a vehicle performing communication between a plurality of road communication stations arranged in a cell and a vehicle-mounted mobile station within the cell.
(1) A road transmission equipment as set forth in claim 1 with the view of achieving the abovementioned object, comprises:
a first transmitting antenna having a directivity in a running direction of a vehicle,
a second transmitting antenna having a directivity in a direction opposite to the running direction of the vehicle, a first transmitting section and a second transmitting section respectively connected to the first transmitting antenna and the second transmitting antenna to output signals of the same frequency, and
a frequency correction section, wherein;
the frequency correction section performs correction so as to provide the first transmitting section with a positive frequency offset for increasing a frequency of a signal supplied to the first transmitting antenna, and so as to provide the second transmitting section with a negative frequency offset for lowering a frequency of a signal supplied to the second transmitting antenna.
In the present invention, an offset for increasing the frequency is provided for an radio wave directed in the running direction of the vehicle while an offset for lowering the frequency is provided for an radio wave directed in the opposite direction to the running direction of the vehicle, for transmission of these radio waves.
Therefore, the variation in the received frequency based on the Doppler shift is reduced for the vehicle-mounted receiving device to lessen the requirements for frequency control of automatic frequency control (AFC). Thus, the degradation of the quality of data after demodulation is reduced.
(2) It is preferred that the amounts of the positive frequency offset and the negative frequency offset provided by the frequency correction section are equal to each other (claim 2).
The reason being that since the running speed of the vehicle is normally almost consistent within the cell, the amount of Doppler shift of the radio wave directed in the running direction of the vehicle, to which the vehicle-mounted receiving device is subjected, is also considered to be the same as that of the radio wave directed in the opposite direction to the running direction, to which the vehicle-mounted receiving device is subjected.
(3) The road transmission equipment according to the present invention may further comprise a speed detection means for detecting the speed of the vehicle running in the cell, wherein
the frequency correction section may set the amount of the frequency offset based on the detected speed of the vehicle (claim 3).
Since the amount of Doppler shift of the vehicle-mounted receiving device can be obtained if the running speed of the vehicle can be detected, the amount of a frequency offset can be set based on the amount of the Doppler shift. Accordingly, in the case where the speed of the vehicle changes with time, accurate frequency correction can be performed in real time.
When a plurality of vehicles are present in the cell and the speed of each vehicle can be detected, the amount of frequency offset is set based on the average value of the speeds of a plurality of vehicles.
(4) The amount of the frequency offset provided by the frequency correction section may be set to a fixed value on the assumption that the vehicle is subjected to constant Doppler shift (claim 4).
Normally, it is considered that the running speed of a vehicle is almost always consistent within the same cell on the same road and does not greatly change with time (although the running speed changes considerably in the case of traffic restriction or traffic congestion, the frequency and the duration of traffic restriction or traffic congestion cannot be predicted).
Therefore, even with the fixed amount of frequency offset, the object of the present invention of reducing the variation in the received frequency based on the Doppler shift can be achieved.
Moreover, since the speed detection means is no longer needed, the configuration of the road transmission equipment is advantageously simplified.
(5) The first transmitting section and the second transmitting section may transmit an orthogonal frequency division multiplex (OFDM) modulated radio wave (claim 5).
In a case where an OFDM modulation method is used for dividing transmitted information into subcarriers and transmitting the obtained subcarriers, the sensitivity of a bit error rate with respect to a frequency disarrangement is high because a distance between the frequencies of adjacent subcarriers is small. Accordingly, in a conventional communication system between a road and a vehicle as illustrated in FIG. 9, a Doppler frequency change increases, thereby degrading the transmission characteristics.
Since the correction for providing a frequency with an offset is performed so as to reduce a Doppler frequency change in the present invention, the present invention is extremely effective for a communication system between a road and a vehicle using an OFDM modulation method.